An organic light emitting diode (OLED) is a self-emissive device using an organic compound as a luminous body. That is, when a voltage is applied to the organic compound, injected electrons and holes recombine in the organic compound, and energy generated by the recombination excitesorganic molecules to emit light. An OLED display device has self-emissive characteristics and advantages of a wide viewing angle, a high definition, and a fast response time. Thus, the OLED display device has attracted considerable attention as a new display device that may replace a liquid crystal display (LCD), and small-sized OLED panels are being produced in large quantities.
OLED display devices are classified into passive matrix OLED (PMOLED) display devices and active matrix OLED (AMOLED) display devices according to a pixel structure. Above all, in an AMOLED display device, each pixel includes at least one transistor functioning as an active device. A scan signal and a data signal are applied to each pixel having at least one transistor. The corresponding pixel is selected in response to the scan signal, and the data signal is applied to the selected pixel. An OLED performs an emission operation at a predetermined luminance in response to the data signal applied to the pixel.
In the above-described AMOLED display device, each pixel includes at least one transistor. In most cases, the transistor is a thin film transistor (TFT) formed on a substrate. The TFT may use an a-Si layer or a poly-Si layer as a channel layer. When the a-Si layer is used as the channel layer, the mobility of carriers is low. Accordingly, a poly-Si layer having a carrier mobility of 10 cm2/Vsec or more is used as the channel layer. In order to form the channel layer using poly-Si, an a-Si layer is formed at a low temperature, and a phase change of the a-Si layer into a poly-Si layer is induced using laser or catalyst. In the above-described process, a plurality of poly-Si channel layers are formed on a panel, thereby forming TFTs in respective pixels.
Also, transistors of an OLED panel must be set to have uniform electrical characteristics in order to realizeprecise image data. In other words, TFTs of respective pixels must be set to have the same electrical characteristics. However, in terms of a fabrication process, it is very difficult to form TFTs having uniform electrical characteristics over the entire area of the OLED panel. When the area of OLED panels gradually increases, a technique of forming TFTs significantly affects the yield of the OLED panels.